Subnet zero and all ones subnet are two different things buth both are co related. Subnet zero means we can use the network ID or network address ( ex. 192.168.1.0 ) as an IP address and we can configure it on our router interface.
All ones subnet means we can also use the broadcast ID or broadcast address ( ex. 192.168.1.255 ) as an IP address and we can configure it on our router interface as an normal IP address.
This feature is enabled on all the new IOS and it is default on all the IOS since 12.0 IOS. If you are still using an old IOS then you can also enable it using the ip subnet 0 global command.
In the past , network address and broadcast address were not allowed to be use as IP address. That time IEEE engineers was thinking that these address can only be used for network ID and broadcast ID. But later they allowed to use these addresses. But still it is recommended that these addresses should only be use on network devices like router or layer 3 switches as well as it should make sure that it will not cause any traffic.
In real environment , I have seen and configured these addresses on loopback interfaces as well as on physical interfaces
Below is the command to enable the Subnet Zero and All Ones Subnet on OLD devices:
Router(config)# ip subnet 0
Below is the example of both subnet zero and all ones subnet:
Router# conf t RouterA(config)# interface serial 0/0 RouterA(config-if)# ip address 192.168.1.0 255.255.255.0 RouterA(config-if)# no shut RouterA(config-f)# exit RouterA(config)# interface loopback 0 RouterA(config-if)# ip address 10.140.1.255 255.255.255.255 RouterA(config-if)# no shut RouterC# conf t RouterC(config)# interface serial 0/0 RouterC(config-if)# ip address 192.168.1.1 255.255.255.0 RouterC(config-if)# no shut RouterC(config-f)# exit RouterC(config)# interface loopback 0 RouterC(config-if)# ip address 10.170.1.255 255.255.255.255 RouterC(config-if)# no shut
In above example we are assuming that IOS is above 12.0. So we have not configured the ip subnet 0 command. But you can configure it to enable this feature.
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